'96 855 Alarm/Immobiliser/Central Lock

Hi,
Can anyone offer advice on the following, I'm not sure if it is a malfunction or a quirk of the Volvo Alarm system (Factory fitted)

Model: 1996(N) 855GLT 2.5 20V Auto. Mileage 51,000.

Following a very intermittent 'fault' with the operation of the alarm/immmobiliser etc. via the key fob I bought a new Volvo Key fob (£53)last week. The responsiveness of the central locking function seemed to be better, still slow compared to my VW Golf but I understand this is due to the way in which the Volvo system is wired.
Today the new key fob didn't work, the car was opened using the key which does not disable the alarm or the immobiliser...lots of beeping and no go.

The car was opened and the alarm /immobiliser disarmed using the spare(original fob).

I reprogrammed the new fob (4 on/offs + 1 on of the ignition), noted that the new one then worked but the original did not, reprogrammed the original, tested both while sitting in the car - both worked, got out the car, the new key fob failed to work the original still works!...Hmmmm.

Are there any alarm experts out there who can advise:
1. Is the system configured such that the alarm/immobiliser can only be disabled using the key fob. This seems illogical as possession of the key should surely 'permit' access and disarming of all anti-theft functions ?
2. Is there a 'work around' to enable the car to be driven should the fob fail to open the car?
3. Does the the failure sequence I have described above give any clues as to what fault may be in the system if any?
4. Could I simply be unlucky and have bought a faulty new fob or is this clutching at straws?
5. Is there any diagnostic function that can be run on the alarm system to help fault find.

The car is used mainly by my wife and with three small children this fault is creating major hassle. I'd appreciate any advice.
Thanks